Direct observation of pulse propagation of femtosecond laser in quartz glass with femtosecond time-resolved optical polarigraphy

摘要

Bulk modifications of various transparent materials such as a quartz glass using femtosecond laser pulses have been received considerable attention recently, because the laser pulse causes easily multiphoton absorption of transparent materials with wide band gaps. Various self-modulating linear and nonlinear effects in the propagation of an intense femtosecond laser pulse in such a dispersive material should be interwined with each other. The effects include the normal and high-order group velocity dispersion, transverse diffraction, self-focusing, self-steepening, self-phase modulation, self-defocusing due to plasma formation and multiphoton absorption. Nevertheless, there has been developed no direct diagnostic technique for propagation of self-modulating femtosecond laser pulses in bulk. Some of the authors demonstrated a direct method for visualizing pulse propagation of a focused femtosecond laser in air. The method of femtosecond time-resolved optical polarigraphy (FTOP) makes use of birefringence induced by the large electrical field of the laser. In this work, direct observation of modulated pulse shapes in propagation inside dispersive transparent materials such as a quartz glass was demon strated successfully.